Neoplastic transformation and tumor progression is mediated by intrinsic genetic changes in tumor cells, as well as extrinsic factors such as infiltrating innate inflammatory cells, adaptive immune cells, endothelial cells (EC), the extracellular matrix (ECM) and mesenchymal-derived stromal cells (MDSCs). MDSCs (tumor associated fibroblasts (TAFs) and pericytes) have emerged as important players in tumor initiation, invasive growth and metastasis through their roles in ECM remodeling, angiogenesis and stromagenesis. However, the mechanisms involved are not yet well-defined. The long-term goal of this project is to define such mechanisms. This proposal is focused on the role of a stromal cell-restricted protease, Fibroblast Activation Protein (FAP), in lung cancer. FAP is a well defined marker of TAFs and blood vessel associated pericytes that can be exploited to study the generation and function of these important cell populations. FAP expression is restricted to fibroblasts in tumors, wound healing and fibrosis and is tightly regulated. FAP contains a structurally defined catalytic domain that mediates its protease activities. Our preliminary data establish that FAP+ cells are a prominent component of human lung cancers and that genetic deletion of FAP indirectly inhibits tumor cell proliferation in mouse models. In addition, our results indicate that FAP is a critical determinant of collagen accumulation in tumors. These data provide proof-of-principle that targeting stromal cell-mediated processes may be an effective approach to treating epithelial-derived solid tumors. We now propose to define the mechanistic bases of the tumor promoting activity of FAP in mouse models of lung cancer (Aim 1) and to define the mechanisms that drive the generation of TAFs (Aim2). In Aim 3, we will test the hypothesis that FAP promotes the growth of human lung tumor cells through mechanisms conserved between mouse and man. The proposed studies will define TAF-dependent mechanisms that promote the growth of lung cancers. However, as FAP-expressing stromal cells are a prominent feature in a wide variety of tumor types, the results of these studies are likely to have broad implications for epithelial-derived tumors. These studies will likely lead to the development of stroma-targeted therapies that are likely to synergize with therapies directed against tumor cells per se.